The FAS 6004 is essentially the same pistol as the earlier 604. When originally introduced in the early 80’s, the FAS 604 was a giant leap ahead of the spring-piston match air pistols in size, weight, balance, firing behavior, and ease of operation. Obviously, the introduction and evolution of PCP’s has long since upstaged the FAS. Still, in the hands of an experienced shooter the FAS pistol is very capable of competitive scores. As an entry-level match pistol, that just happens to be bargain priced, you really can’t beat the FAS. The caveat is: not all 6004’s get the QC inspection. If your 6004 is quirky, there is hope.

Most of the complaints about the FAS 6004 are the result of Chiappa's mass production. The original 604 was fitted and assembled by hand. The 6004 also appears to be hand assembled but the ‘fitting' part seems to be missing. Corrections can be made to undo the Chiappa errors. Not many shooters have machine tools, so I came up with simpler work-arounds.

The original 604 was designed to work with a unique, wide breech seal which was later replaced by a common o-ring that sits deeper in the seal’s groove.

IMO, the o-ring required a bit more finesse with barrel-to-port-face spacing during assembly which FAS accomplished but Chiappa has not. As I see it, to expedite assembly without the fitting steps Chiappa added an aluminum washer/spacer on the barrel to obtain a tight seal. The result of that genius, though, was ditching on the port face without creating a good seal.

Complaints about the pistol farting and/or failing to push some brands of pellets down the bore are partly related to the seal allowing compressed air to blow by it. The other part pertains to the breech chamfer. If you examine the breech of your 6004 with a magnifier and it looks like it was cut by a visually challenged person with a chisel, you have likely experienced the FAS flatulence and pellet sensitivity.

Chamfer roughness on the pistol used in this thread bit into the skirts of some pellets preventing them from seating sufficiently into the barrel with finger pressure. That situation, combined with seal leakage, occasionally failed to launch a pellet.

The first step to joy is to smooth the port face. Use oiled 400 grit wet/dry paper wrapped around a flat block of wood or piece of metal and sand the face smooth. Apply light pressure and circular motion to maintain flatness of the face.

Smoothing the breech chamfer is next after removing the barrel from the top frame. The barrel is retained in the top frame with two screws. Remove them, and the bracket, and the barrel will slide out so you can work on it.

To lap the chamfer you need something similar in shape to the chamfer bevel. The hardware store is your friend. I found an acorn nut that was a close match, and threaded it onto a pencil to make a lapping tool. Use powdered cleanser, Softscrub, whitening toothpaste, pumice powder, wood working ‘rotten stone’, or any other fine abrasive as a lapping compound. The dry abrasives will need to be moistened with oil. Take your time [it isn’t a fast process] and spin the nut back-and-forth in the breech. Add compound as needed, and check your progress frequently so you don’t overdo it.

You want this.

Lastly in this process, knock the sharp edges off the rifling lands with some 400 paper wrapped around a pencil.

Now that you have a smooth port face and refinished breech chamfer, it’s time to re-seat the barrel. The end goal is to close and latch the top without the barrel contacting and scarring the port face. Leave the factory aluminum spacer/band off the barrel and reinstall the barrel in the top frame. Position the barrel clamp and install the two screws. Don’t tighten the screws, just snug them so the barrel can’t move. Make a paper shim with two layers of printer paper and fold it over the port face. With the o-ring removed from the breech groove, close and latch the pistol’s top frame. Now, with a wood dowel and some form of striker like a small hammer, from the muzzle end tap the barrel back against the paper shims. A couple of light taps will get it done. Next, unlatch the top frame and tighten the two clamp screws. Don’t overdo the tightening, the screws thread into soft alloy. With the breech seal back in place, there should be no marking of the port face. If you do see a mark, use a third layer of paper, remove the o-ring and seat the barrel, again.

In my opinion, the breech o-ring is too small. You can compensate for its size by making a shim washer from paper card stock, or purchase an aluminum shim from McMaster-Carr. Their .010” shim works nicely, part 92661A575. Note that McMaster’s shim needs the inside diameter enlarged a bit to fit properly. 400 paper wrapped around a pencil taper with some wrist action will do it. For an improved seal with a much broader contact surface, try a square-profile Viton o-ring from McMaster, part 1170N21. This square o-ring used with the McMaster shim appears to be a very good setup.

If you haven’t paid attention to the compression piston’s felt washer, it can be seen through the small intake hole in the top of the chamber when you draw the piston back. Dampen the felt with 25wt silicone oil to lube piston’s o-ring and obtain maximum muzzle velocity. A good source of the oil is a hobby shop where it is sold as shock absorber oil for RC cars.

At this point, the 6004 should be transformed into the pistol that the original FAS engineers intended it to be.

addendum:A rare problem that should be mentioned is what appears to be a sticking dump valve. I’ve found that lubing the piston’s felt washer every 500 shots with a few drops of the 25wt silicone oil seems to sufficiently lubricate the 6004’s valve. If I see more of that condition, I’ll tackle it and post something.

I should also comment that the wood shaping and surface finish of the adjustable grip that Chiappa offers for the 6004 is very nicely done. Unfortunately, they’ve made the grip too narrow which leaves the fingers dangling in space unsupported. Their ambidextrous grip is of the same finish quality and very much ergonomically superior. I recommend buying the pistol with the ambi and then, if you must have an adjustable grip, apply the money saved toward a Rink grip.

_________________Political correctness is Fascism pretending to be manners.~ Carlin

Last edited by slofyr on Mon Jul 10, 2017 9:01 pm, edited 1 time in total.

Yes a great tutorial. The pencil trick is something anyone having to use a pellet seater can do. It just needs the confidence to remove and replace the barrel and this method looks foolproof. I was interested to see the original 604 breech seal was rectangular section. With sticking dump valve, taking it out and a light polish will help. see my youtube tutorial on the 604.

slofyr, that was a great how-to....a giant step up from the too frequent helpless sniveling here.

How's this for a helpless snivel?

Dear Chiappa,How much would it add to the price for you to implement something recognizable as quality control so that these things work when they get into customers' hands? How much? $10? $25? How many sales would you lose if the retail price went up that much?

The fact that unfinished 6004s get out the door says everything one needs to know about the culture at Chiappa. The gibbons doing assembly and chimpanzees (apologies to our relatives) on final inspection could never get away with their poor performance without executive approval.

Hell, even the Russians (Izh 46) figured out how to sell an inexpensive air pistol that works out of the box!

Suitably tuned up, how easy are they to cycle? I'm wondering how well they would work for juniors (think 10 year old girls in a 40 shot match).

We sometimes have a problem with 12+ girls and boys with FAS and Gamo Compact. We have a 12 minimum age anyway. 10 sounds a bit young for these unless you allow a loader person at the shooting point like we do with a disabled shooter.

I have just finished doing this superb mod demonstrated by Slofyrs. I tackled it slightly differently, using a high quality chamfering tool on the lathe, then polishing with the aforementioned wet and dry paper.My breech face had not yet become damaged so I have left that alone.Finally I counterbored the front face of the piston by a depth of 1mm and squeezed into it a machined alloy disc about the size of small coin of 1.3mm thickness.I glued it in with Loctite bearing fit.Next I reassembled and revealed that the gun would cock but I could tell that I had bottomed out and in danger of straining the mechanics over time if I left it that way. (Dead heading)I removed the piston again and skimmed 0.15mm off the 0.3mm disk face (or about half its thickness) replaced and this time no dead heading.The gun was about 5lbs harder to cock....possibly a bit more but not enough to cause problems.Using the R10 Pellets I have she is now realizing 408 fps avg. with a high of 411 fps and low of 403 fps.This is now the 400 fps gun as advertised.

Howdy to you all,first time poster, great do-it-yourself thread on improving the 6004. My son uses this in 4-H and recently his pressures have been decreasing. I noticed the O-ring had torn on the upper portion and with only one o-ring left from the original Supply I was looking and saw reference to this thread. I've noticed some of the dishing from the barrel on the port face, the chamfering is fine on my barrel chamber, no issues at all there. I may still polish the air port block face and exchange out the square nitron o-ring and shim. Working on McMaster Cars website if I were to order these parts I will get quantities of 10 and 25 which will be far in excess for our needs. Does anybody have a couple of each that they would like to sell me for five bucks and send me in an envelope? (covered, thanks SLOFYR!)

Thanks, Gary

Last edited by GoWyo! on Thu Feb 23, 2017 2:33 pm, edited 1 time in total.

William, if you will never buy anything from Chiappa, why the vitriol on this thread?

Vitriol? That's nowhere near vitriol.

And why the post? Simple, I long ago got sick and tired of companies who do their beta-testing with unsuspecting customers. When I - or anybody else - spends good money for a product he has every right to expect something that works. Unless it's clearly marked:

This POS may be nonfunctional out of the box and if so may require specialized tools and skills that you may or may not possess. Either way, it's not our fault.

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